1. Field of the Invention
The present invention relates to a sensor chip, and more particularly, to a fingerprint sensor chip.
2. Description of the Prior Art
In semiconductor processing, a fingerprint sensor chip is a type of functional chip used as a biometric sensor, sequentially packaged with a fingerprint discriminator. The fingerprint sensor chip comprises approximately ninety thousand metal plates arranged as a 300.times.300 pixel matrix, these plates positioned between an inter-metal dielectric (IMD) layer and a protective layer. Each of the metal plates is used as a sensor plate of the fingerprint chip. When a user touches the protective layer, each of the sensor plates senses the static voltage from the surface contact of the user's finger. The voltages recorded by each sensor plate will vary with the undulating surface of the user's finger, and together they form a pattern of the fingerprint. This pattern can be passed on to image recognition circuits to provide recognition and discrimination of the fingerprint.
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a schematic diagram of the layout of sensor areas 23 on a fingerprint sensor chip 13 according to the prior art. FIG. 2 is a cross-sectional schematic diagram along line 2--2 of the fingerprint sensor chip 13 shown in FIG. 1. The prior art fingerprint sensor chip 13 is formed on a dielectric layer 12 of a semiconductor wafer 10 on which a plurality of rectangular sensor areas 23 arranged in a matrix format are defined. The fingerprint sensor chip 13 comprises conductors 14 positioned on the dielectric layer 12 surrounding each of the sensor areas 23, an IMD layer 15 covering the dielectric layer 12 and the conductors 14, a plurality of metal plates 22 positioned on the IMD layer 15 above each sensor area 23, and a protective layer 24 covering the metal plates 22 and the IMD layer 15. The IMD layer 15 is a complex structure that comprises a spin on glass (SOG) layer 18 and a silicon oxide layer 20. The SOG layer 18 fills the space between each conductor 14 and smoothes the outer-most sides of each conductor 14. The silicon oxide layer 20 covers the dielectric layer 12, the conductors 14 and the SOG layer 18.
Each of the conductors 14 is used as an electrical connection to pattern recognition circuits or any other internal circuitry of the fingerprint sensor chip 13. Each of the metal plates 22 is used as a sensor plate of the fingerprint sensor chip 13. The protective layer 24 is used to protect the internal circuitry of the fingerprint sensor chip 13. When a user touches the protective layer 24, each of the metal plates 22 senses static voltages from the surface contact of the user's finger. Then, the voltage variations across all the metal plates 22 are combined to produce a pattern of the fingerprint.
However, the SOG layer 18 positioned on the outer-most walls of the two adjacent conductors 14 and the surface of the dielectric layer 12 form a groove, which makes the surface of the silicon oxide layer 20 uneven. Consequently, the surfaces of the metal plates 22 formed on the groove are lowered in their centers and raised at their edges. When the user touches the fingerprint sensor chip 13, the distances from the center of the metal plate 22 to the user's finger and those from the edges of the metal plate 22 to the user's finger are not the same. This difference will effect the sensitivity and accuracy of the metal plates 22.
Also, the surface of the protective layer 24 has protrusions and depressions, in relation to the raised edges of each metal plate 22. When the fingerprint sensor chip 13 undergoes a cleaning process with a high-pressure water column, such as is done after mounting on a printed circuit board (PCB), cracks can easily form in the sunken surface of the protective layer 24. These cracks will allow water to permeate into the electric conductors 14 and lead to short-circuiting. Furthermore, during the packaging of the fingerprint sensor chip 13, cracks in the protective layer 24 can cause it to more easily pull away from the surface of the chip, rendering the device inoperable.